1. Field of the Invention
The present invention relates to a constant voltage circuit using a band-gap circuit.
2. Description of the Prior Art
As means for applying a reference voltage to various electronic circuits, there has been conventionally used a constant voltage circuit utilizing a band-gap circuit. FIG. 4 shows one example of this type constant voltage circuit. More specifically, the illustrated constant voltage circuit comprises a band-gap circuit 2 for generating a reference voltage, an error detecting circuit 4 for detecting an error voltage of the constant voltage outputted from the band-gap circuit 2, and an output circuit 6.
The band-gap circuit 2 includes a serial circuit comprising a resistor 21, a transistor 22, diodes 23, 24 and a resistor 25, and a serial circuit comprising a resistor 26 and diodes 27, 28, 29, these serial circuits being arranged in parallel. A bias voltage created by the diodes 27, 28, 29 is applied to a base of the transistor 22. A power source is connected to the band-gap circuit 2 via a resistor 61 in the output circuit 6 and a source voltage Vcc is applied to a source terminal 10. A value of the resistor 21 is set so that the base and collector of the transistor 22 are at the same potential. Assuming now in the band-gap circuit 2 that a current flowing toward the side of the diodes 23, 24 and the resistor 25 through the transistor 22 is I.sub.1, a current flowing toward the side of the diodes 27, 28, 29 therethrough is I.sub.2, resistance values of the resistors 21, 25 are R.sub.1, R.sub.2, the diodes 27, 28, 29 each comprise a transistor, and a base-emitter voltage of each transistor is V.sub.BE, A constant voltage Vreg outputted from the band-gap circuit 2 is given by; ##EQU1## where the magnitudes of the currents I.sub.1, I.sub.2 are set to meet a relationship of I.sub.2 &gt;I.sub.1, k is the Boltzmann constant, q is the quantity of electric charge of an electron, T is a temperature, and kT/q is a constant. By properly selecting respective values of the resistors 21, 25, 26, a temperature coefficient of the constant voltage Vreg outputted from the band-gap circuit 2 can be made zero.
The error detecting circuit 4 includes a differential circuit comprising a pair of transistors 41, 42 of which emitters are connected in common to each other. Connected to the emitter side of the transistors 41, 42 constituting the differential pair is in series a transistor 43 through which an operating current flows to the differential pair. The diode 29 in the band-gap circuit 2 is connected between a base and an emitter of the transistor 43. The diode 29 and the transistor 43 jointly constitute a current mirror circuit so that the current I.sub.2 flows through the transistor 43 and serves as an operating current for the differential circuit. A voltage drop across the resistor 21 due to the current I.sub.1, i.e., a collector voltage of the transistor 22, is applied to a base of the transistor 41 and a base voltage of the transistor 22 is applied to a base of the transistor 42. The base voltage and the collector voltage of the transistor 22 are compared with each other in the differential circuit to detect an error voltage therebetween. Thus, a current depending on increase or decrease of the error voltage flows through the transistors 41, 42 of the differential circuit. In the illustrated case, when the collector voltage of the transistor 22 is higher than the base voltage thereof, a current corresponding to the error voltage flows through the transistor 41. Transistors 44, 45 jointly constituting a current mirror circuit are connected as an active circuit between collectors of the transistors 41, 42 and an output terminal 12, whereby the current corresponding to the error voltage is taken out from the collector side of the transistors 41, 45.
The output circuit 6 includes a resistor 61 and transistors 62, 63 which are connected in a Darlington arrangement. A base of the transistor 62 is connected to the collectors of the transistors 41, 45. Therefore, when a current representing the error voltage flows through the transistor 41, the current is pulled from the base of the transistor 62 and a base current depending on the error voltage flows through the transistor 62. The current flowing through the transistor 41 is multiplied by a current amplification factor of the transistors 62, 63 and then discharged from the source side to the ground side through the resistor 61. At this time, there occurs a voltage drop across the resistor 61 depending on the current flowing through the transistor 63. Therefore, as the current flowing through the transistor 41 increases, the current flowing through the resistor 61 is increased to enlarge the voltage drop and thus lower the constant voltage Vreg. On the other hand, as the current flowing through the transistor 41 decreases, the current flowing through the resistor 61 is decreased to reduce the voltage drop and thus lower the constant voltage Vreg. By so controlling the current flowing through the resistor 61 in accordance with the error voltage, the constant voltage Vreg taken out from the output terminal 12 is stabilized.
Next, FIG. 5 shows another example of the above type conventional constant voltage circuit using a band-gap circuit. In this constant voltage circuit, instead of the resistor 61 shown in FIG. 4, a resistor 64 and a transistor 66 are provided in the output circuit 6 besides the transistors 62, 63, with an emitter of the transistor 62 directly connected to the source line. The constant voltage circuit shown in FIG. 5 can also similarly stabilize the constant voltage Vreg to be taken out from the output terminal 12.
However, the constant voltage circuit shown in FIG. 4 has such disadvantages as that since a load current flows through the resistor 61 and produces large power, the resistor 61 must be outside an integrated circuit, that the current which flows through the transistor 63 for stabilization becomes a reactive current, and that because of the reactive current being large, the transistor 63 is required to have a large capacity. Also, the constant voltage circuit shown in FIG. 5 has such disadvantages as that since the transistors 63, 66 provided in the output circuit 6 each comprise an NPN type transistor, the constant voltage Vreg to be obtained cannot become greater than the value of source voltage Vcc-base-emitter voltage V.sub.BE, and that a large reactive current flows through the transistor 63 as with the constant voltage circuit shown in FIG. 4. Further, either constant voltage circuit requires the source voltage Vcc to be high in raising of the constant voltage Vreg. Another disadvantage of each constant voltage circuit is in that Vreg-Vcc characteristics are changed to a considerable extent depending on temperatures as exemplified at T.sub.1 (=50.degree. C.), T.sub.2 (=75.degree. C.), T.sub.3 (=25.degree. C.), T.sub.4 (=-5.degree. C.) and T.sub.5 (=-25.degree. C.) shown in FIG. 6.